Atelectasis can provoke pulmonary and non-pulmonary complications after general anaesthesia. Unfortunately, there is no instrument to estimate atelectasis and prompt changes of mechanical ventilation during general anaesthesia. Although arterial partial pressure of oxygen PaO2 and intrapulmonary shunt have both been suggested to correlate with atelectasis, studies yielded inconsistent results. Therefore, we investigated these correlations.

Methods

Shunt, PaO2 and atelectasis were measured in 11 sheep and 23 pigs with otherwise normal lungs. In pigs, contrasting measurements were available 12 hours after induction of acute respiratory distress syndrome ARDS. Atelectasis was calculated by computed tomography relative to total lung mass Mtotal. We logarithmically transformed PaO2 lnPaO2 to linearize its relationships with shunt and atelectasis. Data are given as median interquartile range.

Results

Mtotal was 768 715–884 g in sheep and 543 503–583 g in pigs. Atelectasis was 26 16–47 % in sheep and 18 13–23 % in pigs. PaO2 FiO2 = 1.0 was 242 106–414 mmHg in sheep and 480 437–514 mmHg in pigs. Shunt was 39 29–51 % in sheep and 15 11–20 % in pigs. Atelectasis correlated closely with lnPaO2 R2 = 0.78 and shunt R2 = 0.79 in sheep P-values<0.0001. The correlation of atelectasis with lnPaO2 R2 = 0.63 and shunt R2 = 0.34 was weaker in pigs, but R2 increased to 0.71 for lnPaO2 and 0.72 for shunt 12 hours after induction of ARDS. In both, sheep and pigs, changes in atelectasis correlated strongly with corresponding changes in lnPaO2 and shunt.

Discussion and Conclusion

In lung-healthy sheep, atelectasis correlates closely with lnPaO2 and shunt, when blood gases are measured during ventilation with pure oxygen. In lung-healthy pigs, these correlations were significantly weaker, likely because pigs have stronger hypoxic pulmonary vasoconstriction HPV than sheep and humans. Nevertheless, correlations improved also in pigs after blunting of HPV during ARDS. In humans, the observed relationships may aid in assessing anaesthesia-related atelectasis.